The present invention relates to a method of improving the response characteristics of a continuous gas sensor using a microwave spectrometer and, particularly, to such a method whereby continuous measurement of stable isotopes is made possible.
It has been known that the conventional gas sensor utilizing a microwave spectrometer is generally large in size and expensive to manufacture and the handling thereof is relatively difficult.
In view of these facts, an improvement has been proposed to the conventional gas sensor of this type which includes a resonant cavity sample cell and a solid-state microwave oscillator (instead of the conventional klystron), resulting in a gas sensor which is compact in size, inexpensive to manufacture and more sensitive than the conventional sensor.
In the practical use of such gas sensor, however, problems have been uncovered. Particularly, substances to be detected by the microwave spectrometer are polar molecules having a significant vapor pressure. Examples of such polar molecules include nitrogen compounds such as NH.sub.3, N.sub.2 O and NO.sub.2, sulfur compounds such as SO.sub.2, CH.sub.3 SH and COS, aldehydes such as HCHO and CH.sub.3 CHO, and other molecules such as H.sub.2 O. These molecules tend to be adsorbed on walls of the constituent components of the gas sensor with which these molecules contact. This adsorption may lengthen the response time of the gas sensor for variations of concentration of the substance.
FIG. 1 illustrates this fact. In FIG. 1, NH.sub.3 of 20 ppm (the polar molecule) is supplied to the gas sensor. FIG. 1 shows an output signal obtained from the gas sensor. As is clear from FIG. 1, a 90% indication is obtained after 10 minutes from injection of NH.sub.3 of 20 ppm, and a 100% indication, i.e., 20 ppm, is obtained after about 40 minutes from the sample injection. That is, even with this improved gas sensor, it is impossible to use it as a sensor for continuous measurement of minute amounts of polar molecules whose concentration varies rapidly. In other words, in other to quantitatively measure a sample immediately after the injection thereof, it is necessary to inject an increased amount of the sample to the senor, resulting in a substantial degradation of the sensitivity of the instrument.
This problem becomes more severe when polar molecules to be measured contain stable isotopes and are injected successively in gas form from an apparatus such as a gas chromatography apparatus. For example, when polar components contained in the injected material from a gas chromatography apparatus are converted by a preprocessing unit to a chemical form suitable for use in microwave spectroscopic measurement and supplied successively to the sensor, it may become impossible to measure them precisely because one of the components supplied thereto affects the measurement of the components supplied thereto subsequently.
FIGS. 2A and 2B are graphs depicting this fact. In FIG. 2A, .sup.14 N-nitrobenzene and .sup.15 N-nitrobenzene are supplied successively to a preprocessing unit containing Ni as a catalyst to convert them into .sup.14 NH.sub.3 and .sup.15 NH.sub.3 by hydrogenation, and nitrogen isotope labelled ammonia molecules are supplied successively to a sensor tuned to 23,870 MHz, which is the resonant frequency of .sup.14 NH.sub.3.
As is clear from FIG. 2A, .sup.14 NH.sub.3 is measured in response to a first injection of .sup.14 N-nitrobenzene and is also measured in response to a second injection of .sup.14 N-nitrobenzene. So long as the second injection is concerned, it is also measured although there is a considerable tailing of the detected waveform. However, when .sup.15 N-nitrobenzene is injected thereafter, .sup.14 NH.sub.3 is still measured. This phenomenon is repeated for subsequent injections of .sup.15 N-nitrobenzene.
In FIG. 2B, .sup.15 NH.sub.3 is measured, in the same manner as described with reference to FIG. 2A, by setting the resonance frequency of the microwave sensor at 22,789 MHz. The result is similar to that shown in FIG. 2A.
In order to resolve these problems, it may be effective to constitute at least the wall portions of the measuring system which the samples contact with a material whose adsorptivity for the sample is low and/or to reduce the total area of the wall portions substantially. Alternatively, it may be effective to maintain the interior of the measuring system under a balanced absorption condition. These approaches, however, are very difficult to realize in practice.